REMOVAL OF COPPER FROM CARBON-IRON MELTS
Yuri Kostetsky
Alexander Troyansky
Maxim Samborsky
Donetsk State Technical University, Artyoma Str. 58, Donetsk, 83000, Ukraine
Abstract
During a heat or a ladle treatment copper can’t be removed from steel melts via traditional
refining methods. Among other methods, which can ensure copper removing from iron melts,
refining with sulfide fluxes may be highlighted as very promising to be a base of industrial
technology. The slag systems on the base of sodium sulfide and aluminium sulfide are the
most appropriate for application. First stage results of laboratory investigations show that it is
possible to remove copper from iron melts by treatment with soda and soda containing
mixtures.
The accumulation of tramp elements in the scrap, mainly copper, is a widely concerned
problem. Until now the problem of copper removal from steel during the conventional
steelmaking processes has not found an appropriate solution. Usually standards limit copper
content in the finished steel up to 0,2-0,3%. Exceeding this level brings noticeable diminution
of mechanical properties of metal and may account for hot shortness in the hot-rolling
operation, cracks during cold forming and even during continuous casting operations.
Copper comes to steel with the metal part of charge. Almost all of metal materials bring it
(Fig.1) /1/. But really in steelmaking under control is only the copper level in the scrap.
Table 1
Copper contamination of charge materials
Type of charge material
Waste chips
Obsolescent scrap and electric-furnace bundles
Heavy home scrap
Miscellaneous home scrap (scrap, tedges, pours, deadheads)
Ferroalloy
Auxiliary aluminum
Percentage of copper,
%
0,23-0,58
0,14-0,42
0,07-0,10
0,16-0,18
up to 1,2
up to 3
Special sorting and separation during scrap processing allows lowering a copper
accumulation in the steelmaking products. For example cutting up of the scrap in the pieces
up to 300x300 mm instead of 600x600 mm and subsequent copper impurities picking up with
the electromagnetic separator gives additional copper extraction 1,5-2 times as much.
Furthermore it is possible to reduce a harmful influence of copper presence in steel by nickel
or molybdenum addition in quantity close to copper concentration /2/. Rare-earth metal
additions also afford a beneficial effect. But these methods do not solve the problem in whole.
In this connection it is important to develop effective methods of iron melts refining from
copper contamination.
Principal peculiarity of copper as impurity is lower affinity to oxygen than that for iron in
conditions of steelmaking processes. As a result it is not eliminated from liquid metal under
oxidizing conditions during a heat. During a ladle treatment copper is not removed from steel
too. For the last decades numerous ideas have been proposed for the removal of copper from
molten iron /3/. Among them may be selected as promising methods for steelmaking:
copper removal from molten iron into the gas phase by evaporation in vacuum or
!
during the treatment with the aid of special gas mixtures;
filtration of iron melts;
!
Slag treatment with sulfide fluxes.
!
In this work the last method is discussed as the most appropriate and promising for a
commercial application. It is based on the facts that copper has larger affinity to sulfur than
that for iron and on existing of the immiscibility interval between liquid iron sulfite and ironcarbon melt in Fe-C-S system.
A reaction of copper removal from metal to sulfide flux can be represented as follows:
[Cu ] + 1 / 2[ S ] = (CuS 0.5 )
K=
a( CuS 0.5 )
a
1/ 2
[ Cu ] [ S ]
a
=
γ ( CuS 0.5 ) X ( CuS 0.5 )
γ [Cu ] X [Cu ] a[1S/ 2]
(1)
(2)
Where γi and Xi are the activity coefficients and mole fractions of compounds, and ai denotes
the activity of compounds.
An expression for the distribution ratio of copper can be derived from equation (2). After
rearrangement and introducing the mass percent instead the mole fraction of copper in the
metal and flux the desired expression may be written as
LCu =
γ [ Cu ]
(%Cu )
=C
a[1S/ ]2
[%Cu ]
γ (CuS0.5 )
(3)
Where C is the coefficient correcting for the conversion into mass percentages and including
the equilibrium constant of the reaction (1).
The analysis of the expression (3) shows that increasing of sulfur activity and copper
coefficient of activity in metal leads to increase of the distribution ration of copper. Large
growth of sulfur activity is not acceptable because it is tied with increasing of the sulfur
concentration in the metal. As regards for activity coefficient of copper it increases with
growing of carbon and silicon concentrations in metal and with decreasing of metal’s
temperature. However the influence of these factors is negligible from the practical point of
view. Therefore the more preferable way to increase the distribution ratio of copper between
slag and metal is a decrease in value of copper sulfide activity coefficient.
The distribution ratio of copper between carbon-saturated iron and liquid FeS approximately
equal 9 at the temperature 1400oC and sulfur concentration in the metal 1,9% /4/. An addition
of sodium sulfide to FeS leads to increase of the distribution ratio. The maximum value
LCu=24 is observed at the mole fraction of sodium sulfide in slag 0,4 and it slightly changes
due to further increasing of the sodium sulfide concentration. Besides the additions of sodium
sulfide also provide the decrease of sulfur concentration in metal from 1,9% up to 0,04% at
the mole fraction of sodium sulfide 0,8.
Figure 1. Influence of alkali metal sulfide additions to FeS flux onto the distribution ratio of
copper between carbon-saturated iron and slag.
As experiments show additions of another alkali element sulfides and alkaline earth metal
sulfides similarly influence on the LCu magnitude (fig.1) /5/. In laboratory conditions for slag
systems such as FeS-LiS0,5, FeS-KS0,5, FeS-SrS0,5 and FeS-BaS it was fixed follows
maximum values of the distribution ratio 30, 20, 22 and 19, respectively. Additions of Mg and
Ca sulfides did not influence substantially onto LCu value due to their restricted saturation
ability in liquid FeS /6/. Encouraging result was reached for aluminum sulfide, according with
the publications its additions also permit to increase the distribution ratio up to 30 /7, 8/.
Therefore laboratory experiments show the ability to refine an iron melts from copper by a
sulfide slag treatment. The slag systems on the base of sodium sulfide and aluminum sulfide
can be highlighted as the most appropriate for application in commercial processes. As rule
the ideas of such processes employ a counterflow operation to ensure effective interaction
between metal and slag. For instance it was proposed a process for copper removal from
molten iron which uses successive contacts between the Al2S3-FeS flux and metal in the
course of the three stages counterflow operation /8/. This method requires the use of sulfide
resources, aluminum, carbon, flux for desulfurization, electricity for heating, etc., and
specially designed equipment. So it is critical from the economical point of view. Clearly it is
main reason why such ideas didn’t reach an industrial realization until now.
There is data about experiments in which soda was used for a ladle treatment of open-hearth
pig iron /9/. In the experiments pig iron, contains 0,23-0,5% sulfur and 0,23-0,38% copper,
was tapped from EAF into a ladle which was preliminary loaded with soda. It is suggested that
sodium sulfide was produced according the reaction:
Na 2 CO3 + [C ] + [ S ] = Na 2 S + CO + CO2
(4)
After treating copper percentages in pig iron was lower on 0,03%. The same operation gave
more perceptible results under laboratory conditions. Authors explained that through higher
sulfur content in the pig iron (up to 1,53%) during the laboratory experiments. But observed
laboratory results were some unstable. It was tied with features of interaction between melt
and soda. The interaction leads to strong gasification and as result appreciable volatilizing of
sulfides. Thus chose method of the reactant introducing into molten metal can’t be adopt as
effective, but it is interesting to clear up more deeper possibilities of iron melts refining from
copper by soda.
Such investigations were started at the Donetsk state technical university. It is suggested
introduce a refining reagent into liquid metal by powder injection or in the form of granules.
The first stage of the work was devoted to elucidating fundamental possibilities of iron melts
refining from copper by soda or refining mixtures containing soda in a laboratory
environment.
Refining
mixture
Crucible
Bell
Iron
melt
Figure 2 Cheme of powder materials
introducing in the course of the
experimental heats
A master alloy containing copper was
prepared by alloying of pig iron with
copper chips in a graphite crucible in an
induction furnace. Then in each
experiment, 500 or 350g of prepared pig
iron was melted also in a graphite crucible
in a vertical resistance furnace and at
1523K refining materials were introduced
in the iron melt under argon atmosphere.
Like this several series of heats were made.
In the course of experiments follows
materials were examined:
− fused soda block;
− soda and sulfur powders;
− mixtures of soda, sulfur and carbon
powders.
In order to eliminate vigorous metal
splashing and to lower ineffectual losses by
evaporation during a treatment powder
materials were introduced into metal in a
special bell made with graphite or quartz
glass (fig.2).
Figure 3 shows an average percentage change of carbon, copper and sulfur concentration in
the metal reached by a treatment relative to them start concentration for the each series. In the
course of the experiments two different scheme of treatment were employed. In I, II series of
experiments a portion of sulfur was added to the metal, which contains 3,93% of carbon,
0,64% of copper, 0.043% of sulfur, and then a soda block is introduced into the melt within
five minutes. In two minutes a sample for chemical test was taken away. In the other series pig
iron, contains 4,13% of carbon, 0,53% of copper and 0,1% of sulfur, was treated by a mixture
of sulfur, soda and carbon. Samples for chemical test were taken away after five minutes
holding.
Figure 3. Carbon, copper and sulfur average percentage change due to a treatment relative to
them start concentrations for the each experimental series.
Only second variant of the treatment, III-VII series, brought decrease of the copper percentage
in the metal and the best result η[Cu]=24% was observed in VII series. But the results show
some instability in III-VII series. It is responsible for unstable assimilation of refining
materials in the course of experiments due to vigorous gasification. Take attention a clear
correlation between changing of carbon and copper concentrations in the melt, which is
observed after treatment in III-VII series. It is an evidence the reaction (4) takes place. A
principal drawback of this variant of treatment is enough high final percentage of supfur in the
metal.
The sulfur concentration in the metal didn’t change strongly in II series and I. But copper
percentage increased appreciably. Though it is hard to guess that a sulfur potential was too
low for copper slagging after sulfur addition. Approximate mass balance shows that whole
mass of copper in the metal must be lowered on 10% at least. Observed increasing of copper
percentage can be explained by large losses of iron for slagging without subsequent noticeable
copper extraction into the slag. Great sulfide slag losses occurred during treatment by a soda
block.
Thus represented results of the first stage of the investigation may be a point for discussion.
Unfortunately they didn’t give a possibility to make a final decision as for real efficiency of
the examined refining methods with soda, but it brought necessary information for subsequent
experiments planning. A computer model of the physical-chemical processes, which take
place in course of the treatment, is under development now.
LITERATURE
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